In JP-2001-16694-A, an ultrasonic sensor is disclosed which is installed in a vehicle and detects a distance between the vehicle and its surrounding obstacle when the vehicle is being parked or turning. This conventional ultrasonic sensor is shown in FIGS. 8A and 8B and designated with reference numeral 190.
In the ultrasonic sensor 190 in FIGS. 8A and 8B, an inner case 121 with a piezoelectric vibrator 101 attached to its bottom portion 121a is covered by a vibration-absorption body 103 such as a rubber. The inner case 121, the piezoelectric vibrator 101, and the vibration-absorption body 103 are inserted into an outer case 141 having a flange portion 141a. The inner case 121 is a metal can which has a shape of a cylinder having a bottom and is made of, for example, aluminum. The ultrasonic sensor 190 with such a structure is embedded in a hole on the bumper 110 of the vehicle, with the bottom portion 121a facing the outside of the vehicle.
The ultrasonic sensor 190 installed in the bumper 110 transmits an ultrasonic sound by causing, by means of the piezoelectric vibrator 101, the bottom portion 21a to vibrate as a vibration plate, receives at the piezoelectric vibrator 101 the ultrasonic sound reflected by the obstacle, and thus detects the obstacle.
The ultrasonic sound generally used for the propagation in the air has a frequency within a low frequency range, for example, below 100 kHz, because the ultrasonic sound decays faster in the air as its frequency becomes higher. The thickness and the size of the inner case 121 of the ultrasonic sensor 190 are designed so that the bottom portion 121a vibrates at an intended frequency.
Therefore, the requirement for the vibration frequency of the bottom portion 121a makes it very difficult to miniaturize the inner case 121 beyond a certain extent. In fact, the resonance frequency of the inner case 121 becomes higher than the low frequency range if the inner case 121 is made of metal (such as aluminum) like the conventional ultrasonic sensor. The difficulty may be overcome if the thickness of the inner case 121 is reduced. However, there is almost no room for reducing the thickness in view of requirement for avoiding a physical damage on the inner case 121.
In view of this, the inventors of the present invention considered using a plastic case in place of the conventional metal case. The consideration is based on an expectation that an ultrasonic sensor with the plastic case can be miniaturized while keeping its vibration frequency within the low frequency range, because the plastic is less stiff than the metal.
However, the inventors found it necessary to solve problems as follows in using the plastic case as a case of the ultrasonic sensor.
The first problem is related to electric conduction of electrodes of the piezoelectric vibrator. Each of the electrodes is formed on each side surfaces of a piezoelectric substance of the piezoelectric vibrator and is for driving the piezoelectric substance. In the conventional ultrasonic sensor, one of the side surfaces forming the electrodes is attached to the inner case 121 to fix the piezoelectric vibrator 101 and the electric conduction of the electrode attached to the inner case 121 is achieved through the metal inner case 121. In fact, as shown in FIGS. 8A and 8B, one of leads 105 is connected with the bottom portion 121a of the inner case 121. However, the plastic case cannot be used to achieve the electric conduction of the electrode in the same manner as the metal case.
The second problem is that the bottom portion of the plastic case as a vibration plate possibly generates an unwanted resonance at a frequency other than the intended frequency, because the plastic case is less stiff. It is preferable to diminish the unwanted resonance, because it may harm reverberation characteristics and directional characteristics of the ultrasonic sensor.